This invention relates in general to electroanalytical instruments, and in particular to cartridge-shaped reference elements especially useful for potentiometric measuring systems as well as a process for the production of such reference elements. For a discussion of electroanalytical measuring systems, attention is invited to the literature, for example, Principles of Instrumental Analytics, Third Edition, 1985, Douglas A. Skoog, Saunders College Publishing, Philadelphia, pp. 567-713, particularly pp. 600-664, as well as references cited therein directed to reference electrodes.
To produce a reliable and reproducible transition of potential between a metal conductor and an electrolyte solution, usually so-called reference elements are used. The latter are, for example, components of reference electrodes, pH glass electrodes, ion-sensitive meter electrodes or single-rod measuring systems for pH, redox, etc.
In general, the silver/silver chloride reference element is superior to the other known reference elements, such as, for example, the standard hydrogen electrode, the calomel electrode, and the Thalamid electrode. The reasons for the superiority of the silver/silver chloride reference element are: It can be used over a wide range of temperatures from about -30.degree. C. to +135.degree. C., it is physiologically neutral to most substances employed therewith, and the waste materials resulting therefrom can be disposed of easily.
A simple silver/silver chloride reference element consists of a silver wire immersed in a potassium chloride solution. In this connection, a thin silver chloride layer is formed on the silver electrode and a stable potential results, as long as electrical current and changing temperatures have no effect on the electrode.
The stability of the potential of such a simple embodiment is improved by saturating the potassium chloride electrolyte with an excess of silver chloride. For a further stabilization of the potential, the silver wire can be coated with a silver chloride layer either electrolytically and/or by immersion in a silver chloride melt.
For an even further improvement in the stability of the potential and especially to achieve an improved mechanical quality, silver/silver chloride reference elements are produced in cartridge form.
So-called reference cartridges consist of a thin tube and silver wire electrode, embedded in silver chloride. The tube is formed with a chamber for receiving the silver electrode and the solid silver chloride and is sealed at both ends. In this case, one end is sealed gastight and fluidtight to protect the electric connection from corrosion and the other side is sealed in a semipermeable manner to assure liquid contact with the reference electrolyte, for example the KCl solution, and to enclose the solid silver chloride in the cartridge. Such reference cartridges are known, for example, from the catalogs of the Schott Gerate Company, namely No. 387, page 19 of the 3rd quarter of 1986 as well as from no. 3105, page 30 of the IVth quarter of 1990.
In the cartridge, the gastight and fluidtight closure for protection of the electric connection is achieved by platinum wire fused onto the silver wire, to which in turn a plug-in contact can be welded for later bonding. The unit made from silver wire, platinum wire and optionally plug-in contact is fused in the area of the platinum wire in a glass tube used as an element or housing.
The process of the fusing in this case is especially expensive and critical since the platinum wire both has to be positioned for fusion and has to be guided gastight and fluidtight through the fusion point to prevent later corrosion of the plug-in contact. An especially careful treatment is required for the resulting fusion point, which has to be cooled stress-free to avoid the breaking of the glass by rapid temperature change and resultant mechanical stress.
In addition, it is known, e.g., from DT 1,303,322, that silver wire can be melted on the platinum wire instead of the plug-in contact if plug-in contact technology is unnecessary. This embodiment is just as sensitive to produce with respect to the fusion point as those mentioned above; moreover, since bonding by soldering or crimping seems to provide no special advantages, the more economical plug technology is preferred.
DE-OS 33 05 962 discloses generic, cartridge-shaped reference electrodes. In the systems described therein, a half-cell electrode in contact with the reference electrolyte and mechanically shielded toward the outside is in an inner housing, which housing contains a reference electrolyte in its electrolyte space and is connected with the inner space of an outer housing.
In DE-OS 33 05 962 there are utilized varnished silver wires, which are immersed in glass tubes and are insulated only at the end for potential formation. But this technique was not able to contribute to a better stability of the reference potential.
In addition to an electrode in wire form, planar layers of silver as a basis for a reference element have also been described, for example, in DE-PS 3,818,846. Thus, for example, CA 114(1):2998c discloses the coating of a plane substrate with a silver paste to produce electrodes for electrochemical one-time sensors (thermistors). But a design with the advantages of a cartridge for practical use thus far has not been achieved.